1. Field of the Invention
The invention relates to a device for transmitting a series of data words representing a digitized analog signal, which device comprises modifying means constructed to modify the series of data words, prior to transmission, in dependence upon an additional information signal in such a way that after modification the additional information signal is represented by bits of low significant of the selected data words, the values represented by the modified data words being substantially equal to the values represented by the relevant data word prior to modification.
The invention further relates to a device for receiving data words for use in conjunction with such a transmitting device, which receiving device comprises means for recovering the additional information signal from said bits of low significant of selected received data words.
2. Description of the Prior Art
Such devices are known from the European Patent Application EP-A-0,205,200.
EP-A-0,205,200 corresponding to U.S. Pat. No. 4,750,173 issued Jun. 7, 1988 and assigned to the same assignee as this application. Therein is described a transmission system in which, said application describes a transmission system in which for the transmission of the additional information signal, one or more of the least significant bits of every n-th signal sample of a series of digitized audio signal samples is or are replaced by bits of the additional information signal. After the series of signal samples thus modified has been transmitted the additional information signal is recovered from the signal samples, the original audio signal being recovered from the signal samples by digital-to-analog conversion.
The substitution of the least significant bits in every n-th signal sample introduces a minor error in the signal values. If the logic value of the additional information signal remains constant for a longer time repetition patterns may arise in the errors, resulting in audible whistle tones in the recovered audio signal. This is caused by the fact that the errors are intercorrelated if the additional information signals do not vary significantly.
In the known method the occurence of annoying whistle tones is precluded in that, after recovery of the additional information signal, the signal sample bits used for the transmission of the additional information signal are replaced by a bit of an arbitrary logic value, which substantially reduces the correlation in the consecutive errors. However, this known method has the drawback that additional circuits are needed in the signal processing path where the received signal samples are converted into the original audio signal. This is undesirable, in particular if the number of receiving devices is substantially larger than the number of transmitting devices, as for example in the case that the information is transmitted via a record carrier, such as for example a magnetic tape or an optically readable disc.